1. Field of the Invention
This invention relates to systems and methods for protecting privacy encoded electronic information signals.
2. Description of the Prior Art
In the operation of many two-way radio communication systems, as well as other communication systems, it is a requirement that unauthorized reception and detection of certain sensitive information messages be prevented. In sending such sensitive information signals throughout a system, particularly a large system such as used by a metropolitan police department, it is necessary that the signal remain in a secure format while it is being passed through a large number of system components. A secure signal format should be maintained while the signal is being transmitted, for example, from a portable unit to a satellite receiver, through a repeater, through a base station receiver, through telephone lines linking the aforesaid components, and through audio interconnect and patching systems. Many analog voice privacy systems having limited security are available such as those using frequency inversion, band splitting, and other types of analog signal scrambling. It has been found that a high degree of security is obtained in a digital scrambling system in which a voice information signal, for example, is first analog-to-digital converted with the digital signals resulting therefrom being then encoded into a scrambled form by using shift registers and various gates to thereby produce a pseudorandomly encoded, or scrambled, signal.
Various digital scrambling systems are available, which are useful only for point-to-point, single path communications such as, for example, from a portable to a portable unit, from a mobile to a base unit, or from a base to a base unit. These systems have no facility for multisystem interconnecting. In order for a scrambled message to be transmitted throughout a system it is often necessary to decode, or unscramble, the message to the original message format before reencoding, which provides a potential access point for an intruder into an otherwise secure communication network. When it is desirable to change the scrambler/unscrambler code keys in such a system, it is necessary to insert the new codes at each equipment site, a time-consuming, inconvenient procedure.
Scramblers for a mobile or a portable system are generally considered accessories to such systems and do not readily adapt themselves to full system operation. Portable radios, in particular, have not been widely adapted for use in protected communication systems. In order for these portables to be used most effectively they must be used in systems which have satellite receiver voting systems, that is, systems which select the best received signal from a plurality of receive sites and send that best signal to a central location. Digital scrambling systems are generally not compatible with the analog voting system capabilities used in some satellite receiver systems of the prior art. Many prior art scrambler accessories require signal bandwidths much greater than available in standard communication system equipment. These accessories cannot be used in multi-equipment linked systems having standard system bandwidths and components.
Another problem in the prior art is that because the digital scramblers do not use standard channel bandwidths for transmitting signals, it is not possible to intermix scrambled and non-scrambled equipment in a system effectively. Thus, separate dedicated radio channels for scrambled and unscrambled are required. Prior art digital scrambler systems, thus, do not lend themselves to transparent operation, that is, operation in which the signal being transmitted is handled properly by the system components whether the signal is in a scrambled form or otherwise.
Finally, prior art digital scramblers used in systems are not capable of automatically selecting between either an ordinary audio channel when an ordinary audio signal is present or a digital scrambler channel when a scrambled signal is present. This ability of a communication system to automatically determine whether a channel has a clear voice, that is, an unscrambled signal present thereupon or a coded, or scrambled, signal present thereupon is desirable in order to eliminate the necessity of a message receiver having prior knowledge of the form of a message about to be received.
Consequently, the need for an improved digitally protected privacy coded electronic communication method and system exists.